Oscillators are used in a wide variety of electronic devices to generate a frequency output. In many applications, the frequency is utilized to derive other reference and clock signals for essential electronic circuitry. Therefore, the output of the oscillator needs to be accurately set to a desired frequency. However, most of the time, the generated frequency output comprise a nominal frequency output which requires to be adjusted. For example, in a radio receiver the local oscillator(s) may require tuning to receive at a specified receiver frequency. Therefore, nominal frequency output of the oscillator is adjusted via a tuning means to tune the radio receiver. Oscillator circuits which are tunable by mechanical means are known. Usually, these oscillators utilize a tunable inductor which includes a threaded slug. The inductance is varied by axially moving the slug through the inductor by for example a screw driver. However, mechanical tuning of the oscillators is a cumbersome process which requires human interaction. With the present trend toward automated manufacturing processes, human interaction is not desired and more and more processes require elimination of the human factor. Electronic tuning of the oscillator eliminates the human interaction therefore it is desired to electronically tune the oscillators.
Additionally, in some applications such as radio frequency synthesizers, the output of the oscillator is modulated before application to other synthesizer circuitry. Conventional, modulation circuitry utilizes a varactor which varies the reactance of the oscillating circuitry in accordance with a modulating signal. The varactor circuits, however, require complex biasing networks.
Accordingly, there is a desire to provide a circuit for generating a variable frequency output which overcomes the drawbacks of the conventional oscillator circuits.